1. Field of the Invention
The present invention relates to a semiconductor device having a Schottky junction, and more particularly, to a Schottky diode using a wideband gap semiconductor device.
2. Description of the Related Art
Exhibiting better thermal stability and higher thermal conductivity than silicon, silicon carbide realizes operations at a high temperature and is suitable to a reduction in size and an increase in device density. In addition, since a breakdown voltage of silicon carbide is approximately ten times as large as that of silicon, it is possible to obtain a high capability of blocking a voltage (reverse-direction breakdown voltage).
FIG. 5 is a cross sectional view of a Schottky diode having a conventional structure generally denoted at 500. The Schottky diode 500 includes a silicon carbide substrate 501, and a Schottky electrode 502 of platinum is formed in the top surface of the silicon substrate 501. On the other hand, an ohmic electrode 503 of nickel is formed at the bottom surface. The ohmic electrode 503 is connected onto a conducting plate 505, an external main electrode, by a solder layer 504. The conducting plate 505, made of copper, also functions as a heat-sink.
A bonding wire 506 of aluminum is welded on the Schottky electrode 502 by ultrasonic pressure bonding. The welding of the bonding wire 506 is performed after the silicon carbide substrate 501 is fixed to the conducting plate 505. The Schottky electrode 502 is connected with the other external main electrode (not shown) via the bonding wire 506.
In the Schottky diode 500, since the breakdown voltage is high as described above, the thickness of the silicon carbide substrate 501 needed to obtain a predetermined reverse-direction breakdown voltage (voltage blocking capability) may be thinner. This allows shortening the distance between the electrodes 502 and 503, and hence, largely reduce a current-carrying loss (steady-state loss) at turning the power on.
The breakdown voltage above is dependent upon the width of a depletion layer in the vicinity of the Schottky junction, and the width of the depletion layer is largely influenced by a surface state of the silicon carbide substrate 501.
In the Schottky diode 500, at a step of welding the bonding wire 506 on the Schottky electrode 502, the silicon carbide substrate 501 is subjected to stress in the vicinity of an interface with the Schottky junction. Hence, the surface state of the silicon carbide substrate 501 changes to reduce the reverse-direction breakdown voltage. As a result of this, it has been impossible to achieve a designed voltage blocking capability.
In contrast, if the pressure for welding is reduced in order to decrease the stress upon the silicon carbide substrate 501, a contact resistance between the Schottky electrode 502 and the bonding wire 506 becomes large and a current-carrying loss increases while the bonding strength decreases. This resulted in a damage induced by thermal stress.
Accordingly, the present invention aims at providing a Schottky diode realizing a predetermined reverse-direction breakdown voltage even if welding of a bonding wire changes a surface state near an interface with a Schottky junction, and a method of manufacturing such a Schottky diode.
The present invention is directed to a semiconductor device having a Schottky junction. The semiconductor device includes a semiconductor substrate of the first conductivity type. A well region of the second conductivity type is formed in the top surface of the semiconductor substrate. A Schottky electrode is formed on the top surface of the semiconductor substrate and has a Schottky junction with the semiconductor substrate. Also, a connecting conductive member is electrically connected on the Schottky electrode. The connecting conductive member is selectively connected with the Schottky electrode above the well region. In this semiconductor device, a depletion layer is formed in the vicinity of the pn junction interface between the semiconductor substrate and the well region. Also, the connection surface between the connecting conductive member and the Schottky electrode is included in the top surface of the Schottky electrode located above the well region. Since this depletion layer is not affected by connection of the connecting conductive member, it is possible to obtain a desired reverse-direction breakdown voltage.
The present invention is also directed to a method of manufacturing a semiconductor device having a Schottky junction. The method includes a step of preparing a semiconductor substrate of the first conductivity type, a step of forming a well region of the second conductivity type in the top surface of the semiconductor substrate, a step of forming a Schottky electrode on the top surface of the semiconductor substrate, and a connection step of electrically connecting a connecting conductive member on the Schottky electrode. The connection step includes a step of selectively connecting the connecting conductive member with the Schottky electrode above the well region. Using such a method, it is possible to prevent a decrease in reverse-direction breakdown voltage at the step of connecting the connecting conductive member.